The present invention relates to an internal combustion engine comprising a turbocharger, which is variable in flow rate, and a variable intake valve.
Turbochargers, of which an exhaust turbine recovers, as rotational energy, thermal energy of exhaust gases discharged from an internal combustion engine, and in which a compressor provided coaxially with the exhaust turbine is used to compress an intake air of the internal combustion engine, are frequently used in conventional internal combustion engines. Since a turbocharger is mounted to enable a high output as compared with internal combustion engines, in which no turbocharger is mounted, and a cylinder volume required to realize the same output performance can be set to be small, it is possible to achieve a decrease in frictional loss and pumping loss, thus enabling reduction in fuel consumption.
Under conditions of quick acceleration from low speed, however, sufficient supercharging cannot be performed by virtue of a turbine rotating speed not rising, so that deterioration in operability, which is called turbo-lag, becomes frequently problematic. For the problem of turbo-lag, a turbocharger, in which flow rate-supercharging pressure characteristic is made small in flow rate, is used to cause exhaust pressure to rise quickly even in small flow rate at the time of low speed whereby it is possible to avoid the turbo-lag. However, internal combustion engines with such turbocharger of small flow rate involve a problem that an excessive increase in exhaust pressure under conditions of large flow rate at the time of high rotation or the like brings about a decrease in output, which is accompanied with an increase in pumping loss, and degradation in knocking performance, which is accompanied with an increase in residual gases.
In order to cope with such problem, JP-A-10-220256 discloses a technology, in which a turbocharger having relatively large flow rate-supercharging pressure characteristics is mounted on an internal combustion engine to avoid various problems caused by an excessive increase in exhaust pressure and a variable intake valve is controlled to avoid influences of turbo-lag, which is deteriorated in accordance with an increase in flow rate. Further, JP-A-10-220256 discloses a technology, in which mirror cycle is performed by controlling a variable intake valve to make an actual compression ratio smaller than an actual expansion ratio when a waste-gate valve is opened in large flow rate under conditions of medium-high speed operation, thereby avoiding knocking.
With the technology disclosed in JP-A-10-220256, since a turbocharger of relatively large flow rate is used, conditions of acceleration from low speed are conditions of low flow rate for the turbocharger under, so that sufficient supercharging cannot be performed. With an internal combustion engine, in which a turbocharger is mounted to decrease a cylinder volume of the internal combustion engine, it is necessary to rapidly realize an output performance corresponding to a state of supercharging. Therefore, in the case where torque required to an engine is large as at the time of acceleration, there is caused a problem that an intake air quantity required to realize a required torque cannot be ensured only by controlling a variable intake valve and an output performance cannot be satisfied. Also, with the system of the publication, in which a turbocharger having fixed flow rate-supercharging pressure characteristics is used, when a waste-gate valve is opened in large flow rate under conditions of medium-high speed operation, an intake valve closing angle cannot be changed much for a required intake air quantity since control of an intake valve has considerable influences on an intake air quantity even in the case where an intake valve closing timing is controlled to avoid knocking while mirror cycle is performed. Therefore, there is caused a problem that a fuel saving benefit owing to mirror cycle and an effect of avoiding knocking cannot be produced adequately. Further, supercharging pressure is liable to become unstable under conditions of low flow rate for a turbocharger and so the technology disclosed in JP-A-10-220256, in which such region is frequently used, involves a problem that it is difficult to accurately control torque.